Evidence for a role of the sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase in thapsigargin and Bcl-2 induced changes in Xenopus laevis oocyte maturation

Abstract

Thapsigargin (Tg), a selective inhibitor of sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase (SERCA), causes depletion of intracellular Ca²⁺ stores, hence activation of capacitative Ca²⁺ entry (CCE). Incubation of Xenopus laevis oocytes with Tg resulted in an increased rate of progesterone-induced meiotic maturation. Non-mitochondrial ⁴⁵Ca²⁺ uptake by SERCA-containing microsomes prepared from control wild-type oocytes microinjected with sterile water was inhibited essentially 100% by Tg. However, overexpression of Bcl-2, an oncogene known to protect against Tg-induced apoptosis in certain cell types, resulted in only 40% inhibition of microsomal ⁴⁵Ca²⁺ uptake by Tg while non-inhibited ⁴⁵Ca²⁺ uptake remained unchanged. Moreover Bcl-2 overexpression also protected against inhibition of CCE. I_Cl(Ca) was similar in Bcl-2-overexpressing and control oocytes when intracellular Ca²⁺ store depletion was induced by microinjection of inositol 1,4,5-trisphosphate (InsP₃) and other means and when CCE was induced by means independent of SERCA inhibition. Our data indicate that Bcl-2 affects neither the InsP₃ receptor nor Ca²⁺ entry itself. At the end of a 24-h period after progesterone addition to the medium, only 25% of Bcl-2-overexpressing oocytes had matured compared to 85% of control oocytes. Our data suggest that SERCA participates in Xenopus oocyte maturation by controlling cytosolic Ca²⁺ and/or intracellular Ca²⁺ stores, hence CCE. An observed progesterone-dependent protein kinase-catalysed phosphorylation of SERCA is further indication of its role in oocyte maturation.